3.2.5. Thermal Phase Stability and Hardness after Annealing

The thermal stability of arc-deposited (CrAl)N and (AlCr)N coatings has been investigated through heat treatment in an Ar atmosphere with a hold time of 2 h at temperatures between 600 and 1300 ◦C in combination with XRD analysis [61]. The fcc structure was found to be stable up to 800 ◦C. Three reactions were observed sequentially for the (AlCr)Nsystem:


Above 900 ◦C, the entire coating transforms to a mixed-phase structure. No phase transformation occurred in the hcp-AlN coatings, as they were already in their thermodynamically stable state. This is shown schematically in Figure 10.

The effect of the annealing on the hardness of the same coatings was characterised as well, see Figure 11.

**Figure 11.** Microhardness for (CrAl)N, (AlCr)N, CrN and AlN coatings after annealing at different temperatures, redrawn after [61], original © Elsevier.

The highest hardness was shown for (Cr54Al46)N and (Al71Cr29)N, and was retained up to 800 and 900 ◦C, respectively. The subsequent drop in hardness at higher annealing temperatures is correlated with the phase decomposition [61].

The influence of the coating stress at the decomposition temperature was investigated by varying the deposition temperature. The coating deposition was performed by arc using Al70Cr30 cathodes. It was shown that the decomposition temperatures of the metastable fcc-(Al63.5Cr36.5)N phase depends significantly on the stress level in the coatings. The decomposition process starts at the same compressive stress level of around 4.3 GPa for all coatings that were investigated [75].

Figure 12 shows a comparison of the hardness after annealing at high temperatures for different commercial hard coatings, showing that the (Al70Cr30)N coating has the highest hot hardness for temperatures exceeding 950 ◦C.

**Figure 12.** Hardness after annealing of commercial TiCN, (Ti50Al50)N, (Al66Ti34)N and (Al70Cr30)N coatings on cemented carbide.

### 3.2.6. Oxidation Behaviour

The oxidation behaviour of PVD hard coatings is an important property for applications such as dry high speed cutting and high temperature stressed components such as turbochargers. In a study of Cr-rich (CrAl)N sputtered coatings (Cr/Al = 2.94), a relatively small Al content (Cr44Al15N40 at.%) improved the oxidation stability compared to CrN [15]. The first publication about the oxidation characteristics of arc-deposited coatings (AlCr)N with a Al/Cr ratio > 0.5 showed a lower oxidation rate in comparison with CrN [19], as has been confirmed in further studies [51,61,76,77]. Oxidation tests of (CrAl)N and (AlCr)N coatings deposited by activated reactive evaporation showed excellent behaviour [32]. The authors stated that the investigated (CrAl)N and (AlCr)N coatings exhibited significantly better oxidation behaviour than TiAlN coatings, which has been confirmed in further studies [40,78]. A systematic comparison of the oxidation behaviour of arc-deposited (AlTi)N and (AlCr)N coatings, including long time exposure, was performed in [40]. It was shown that the oxidation resistance of both coatings improved with an increase in the Al content. The oxidation resistance of (AlCr)N coatings was significantly superior to that of (AlTi)N coatings. Figure 13 shows the oxide layer thickness of CrN, (CrAl)N and (AlCr)N coatings after annealing in the temperature range of 800 to 1000 ◦C in an ambient atmosphere for 30 min in 50 and 100 ◦C intervals. The addition of 20 at.% of Al to CrN deferred the start of oxidation by 100 ◦C and reduced the oxidation rate. The onset temperature for oxidation increased and the oxidation rate decreased with a further increase in the Al-content up to 71 at.% Al. At a higher Al content, significantly decreased oxidation resistance was detected for a coating with 83 at.% Al and for AlN. Oxidation then starts already at 800 ◦C. These coatings already have an hcp structure. The negative influence of the hcp structure on the oxidation resistance was confirmed by [79].

A model of the oxidation behaviour for both the onset temperature and the oxidation rate depends on the coating composition and the related phase structure, as described above. However, it was observed in general that Cr and Al ions diffuse to the surface, forming a dense oxide layer acting as a diffusion barrier, thereby limiting the inward diffusion of oxygen [32,76,80]. The onset of oxidation of fcc (Cr,Al)N and fcc (Al,Cr)N always starts with the dissociation to h-Cr2N and nitrogen in the coating. The presence of thermally stable Al–N bonding in the fcc-(Cr,Al)N structure can suppress the reduction of nitrogen in the coating. A dense (CrAl)2O3 or (AlCr)2O3 oxide layer (either amorphous or crystalline) is formed at an early stage of oxidation [76]. This can act as an effective diffusion barrier hindering the inward diffusion of the oxygen. All further reactions are influenced by the Al content at a given temperature.

**Figure 13.** Thickness of oxide layer for CrN, (CrAl)N and (AlCr)N coatings after annealing at different temperatures in an air atmosphere, redrawn after [61], original © Elsevier.

It should be mentioned that there are several ways to increase the oxidation stability of (CrAl)N and (AlCr)N coatings, e.g., by means of synergistic alloying of the coatings with small amounts of Si [81], see also Section 4.4. By way of illustration, Figure 14 shows the excellent oxidation resistance of (Al70Cr30)N coatings in comparison to industrial-standard PVD coatings.
